Hollow fiber membranes are widely employed for industrial uses from microfiltration to ultrafiltration, and polyethylene, cellulose acetate, polysulfone, polyvinylidene fluoride, polycarbonate, polyacrylonitrile, etc. are used as materials for the membranes. Since conventional hollow fiber membranes made of these materials have been developed for the purpose of improving filtration performance, these hollow fiber membranes are low in breaking stress and breaking elongation and are apt to break due to abrupt changes in temperature and changes in pressure at back washing. Various attempts have been made for a solution to these problems, and there is suggested a method of increasing the density of a polymer in the whole hollow fiber membrane by increasing polymer concentration in a membrane-forming solution in the invention disclosed in JP-A-59-228016. However, according to this method, the strength of the membrane is improved, but the pore diameter of the membrane decreases and the water permeability of the membrane greatly decreases. Thus, hollow fiber membranes well-balanced in strength and water permeation performance have not yet been obtained.
On the other hand, a method of increasing the pore diameter of a membrane is generally employed for improving the water permeation performance of a membrane, but an increase in pore diameter generally causes a deterioration in the fractionation performance of the membrane and in the strength of the membrane.
As mentioned above, high-performance hollow fiber membranes well-balanced in strength, water permeation performance and fractionation performance have not been obtained by conventional techniques. For example, JP-A-4-260424 proposes a method for the production of membranes high in strength and excellent in water permeation performance, but membranes produced by this method have a large pore diameter and are not well balanced in water permeation performance and fractionation performance.